Recently, UVIJ recording systems which use UV-setting inks have been developed as ink jet recording systems which can record images on base materials having no particular ink image receiving layer. Presently, their improvements have been made vigorously.
The UV-setting inks are loosely divided into two solventless UV ink which hardly contains non-hardening solvent and water-based UV ink in which a photo-induced polymeric compound is dissolved or dispersed in a water-based medium.
The solventless UV ink has merits of fast recording and already been in practical use since the ink can be hardened and dried simply by exposure to UV light and that the ink produces no harmful substance such as VOC.
However, the solventless UV ink does not shrink during hardening and leaves mounds of ink fixed on the recording paper. This unlevelling can be smoothed to some extent by optimization of UV exposure timing and improvement of ink properties, but dot blurring and excessive glossiness will make the printout different from what is obtained by regular printing.
Meanwhile, the water-based UV ink has features that the water-based UV ink can use polymeric compound of higher viscosity than the solventless UV ink since the water-based UV ink using a water-based medium is diluted by a water-based solvent, that the water-based UV ink can reduce the ink volume and improve the image quality on the base material by evaporating the water medium from the UV-set ink, and that the water-based UV ink is available to the thermal type ink jet recording method.
If the base material is insufficient in the absorptivity of water-based media, the base material may cause dot-blurring, make the UV-hardened ink film fragile, and increase the load to dry the water-based medium. Contrarily, if the absorptivity of water-based media is too high, the ink component and the polymeric component go deeper into the base material and the image density cannot be enough. Further, the ink in the deep part of the base material cannot be fully hardened because the ultraviolet rays cannot reach there. Unhardened ink contains acrid monomers which are harmful to human bodies.
The above features and problems of the UV ink jet recording system are, for example, outlined by “Outlook of UV ink-jet technology” 2002 (“Color materials,” p. 75, (8), and p. 394-p. 400).
In any case, since the water-based UV ink can reduce the volume of the printed ink, the images printed with the water-based UV inks are less textured than those printed with the solventless UV inks. However, to make the best of the features of the water-based UV ink, any contrivances are required judging from the affinity with the base materials. Without them, the water-based UV ink is hard to be put into practical use.
Japanese Non-Examined Patent Publication 2000-117960 discloses a printing method using water-based UV inks whose contact angle is 60 degrees or less to a base material which absorbs no or little ink in order to form high density, glossy, and color-saturated images.
However, the above patent document discloses an example of applying ultraviolet rays to jetted inks a few seconds later after the ink droplets hit the surface of a base material. This example cannot be free from image blurring due to mixture of ink droplets. Further, this example requires only base materials that absorb no or little ink. Furthermore, this example has a great drying load to remove solvent components of the ink.
[Patent Document 1]
Japanese Non-Examined Patent Publication 2000-11796.
[Non-Patent Document 1]
Outlook of UV ink-jet technology 2002 (“Color materials,” p. 75, (8), and p. 394-p. 400))